cache: rule by index requires full cache

cache: rule by index requires full cache

In preparation for on-demand cache population with errors, set on
NFT_CACHE_FULL if rule index is used since this requires a full cache
with rules.

This is not a fix, index is already fetching a full cache before this
patch.

But follow up patches relax cache requirements, so add this patch in
first place to make sure index does not break.

Tested-by: Eric Garver <eric@garver.life>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>